Ceramic monoblock radio-frequency (RF) filters provide for the attenuation/rejection of RF signals with frequencies outside of a particular frequency range or band and little rejection/attenuation to RF signals with frequencies within a particular range or band of interest.
These filters most typically take the form of a six sided block of ceramic material having a plurality of resonators/poles in the form of through-holes extending through the interior of the block and terminating in openings in the opposed top and bottom surfaces or sides of the block such as, for example, as shown in U.S. Pat. No. 4,431,977 to Sokola et al. and U.S. Pat. No. 4,692,726 to Green et al, the disclosures and descriptions of which are incorporated herein by reference.
The bandpass of such a ceramic monoblock filter can be designed for specific bandpass requirements. Typically, the tighter or narrower the bandpass, the higher the insertion loss becomes, i.e., an important electrical parameter. A wider bandwidth, however, reduces the filter's capacity to attenuate/reject unwanted frequencies, i.e., frequencies which are known in the art as rejection frequencies.
Moreover, the reactive RF signal coupling between adjacent and non-adjacent resonators, and thus the level of rejection outside of a bandwidth and performance of such filters is dictated at least to some extent by the physical dimensions of each resonator, by the orientation and location of the resonators relative to each other, and by aspects of the top metallization pattern that is applied to the top surface or side of the block of the filter. Interactions of the electric and electromagnetic fields within and around the resonators and the block are complex and difficult to predict.
Currently, increased levels of rejection of the RF signal outside of a filter's bandwidth and thus improved filter performance can be achieved by adding resonators to the ends of the block. Increasing the length or size of the block however is not desirable in applications where space is limited on, for example, a customer's motherboard.
The present invention is directed to a ceramic monoblock RF filter providing increased levels of rejection outside of a filter's bandwidth and increased performance through the use of structure which in one embodiment comprises grounded RF signal blocking through-holes located and positioned in the filter in a manner that blocks unwanted RF signal coupling between resonators but does not require an increase in the length or size of the RF filter.